There has been a known technique of providing an NOx selective reduction catalyst (which will be sometimes referred to simply as an “NOx catalyst” hereinafter) as an exhaust purifying catalyst in an exhaust passage of an internal combustion engine. The NOx catalyst has the function of reducing NOx contained in the exhaust gas mainly using ammonia as a reducing agent. Various techniques for detecting deterioration of the NOx catalyst have been developed.
In the technique described in Patent Document 1, an abnormality of the NOx catalyst is determined based on the difference between the NOx purification rate at a time when the activity of the NOx catalyst is relatively low and the NOx purification rate at a time when the activity of the NOx catalyst is sufficiently high.
In the technique described in Patent Document 2, reducing agent is supplied to the NOx catalyst until ammonia leaks to the exhaust passage downstream of the NOx catalyst at a time when the temperature of the NOx catalyst is in a predetermined temperature range that is lower than the NOx removal temperature range and included in the ammonia absorption temperature range. A determination as to whether the NOx catalyst is deteriorated or not is made based on the total amount of reducing agent supplied to the NOx catalyst.